It has long been known in the paper-making industries to make the dewatering of a fiber web led from the head box of a paper machine on to a forming wire, more efficient by various dewatering means. Examples of such dewatering means are suction boxes, drainage foils, and suction rolls. These dewatering means are positioned under the forming wire so that the wire moves over the surface of the dewatering means.
Also, it is also known in paper technology that suction rolls and drainage foils rotating at the same speed as the wire create on trailing side thereof a partial vacuum which is proportional to the square of the speed of the wire moving over the dewatering means. The resulting suction effect is utilized for dewatering a fiber web within the wire section of the paper machine.
In suction boxes widely used in paper machines, the suction which removes water from the fibre web is determined by the choice or the adjustment of a suction pump or a fan. In order to prevent wearing of the forming wire due to the suction effect, a wear wire or band is often inserted between the forming wire and the suction box, which wear wire or band moves at the same speed as the forming wire.
A disadvantage of this is that it is rather difficult to thus create a different partial vacuum for different kinds of fiber webs. It has been suggested previously that for example the fan of the suction box should be adjusted. However, this kind of adjustment is difficult to carry out, because a manual adjustment requires a high professional skill from the person carrying out the adjustment, and an instrumental adjustment, in turn, is complicated and therefore expensive and, further, requires regular service. It has also been suggested previously that the partial vacuum created by the foil should be adjusted by varying the angle at which it is positioned with respect to the wire. This adjustment, too, is difficult to carry out, and it has not, either, provided the best possible result because the foils are easily deformed, e.g., when they get worn or dirty, so that the original adjustments set for a new foil do not provide the desired result.